^Well, Chapel gave up a career in bioresearch and became a Starfleet nurse so that she could search for Roger Korby. I suppose it's possible Ogawa could've similarly put MD training on hold to become a nurse, but that would be kind of odd to happen twice.
The thing is, there's a tendency in our society to see nurses as just subordinate doctors, as if doctor is the next step up from nurse, but that's a myth. Doctors and nurses are parallel and complementary professions with different demands. It's not unheard of for nurses to become doctors, but it's not an automatic step, and it's a lateral career move, not a promotion.